The present invention relates to electrical test devices and, more particularly, to a device for testing receptacle ground fault circuit interrupters. A typical power receptacle often includes three female connectors designed to accept a three prong male plug of an electrical device or appliance. The three prongs may be referred to as the "hot," "neutral," and "ground" prongs. The power receptacle, in turn, interconnects the three prongs to the hot, neutral, and ground lines, or conductors, in a local electrical system.
In a typical power circuit in the United States, the power at a receptacle in a home or office is supplied at 110 volts, root-mean-square ("RMS"). The voltage at the "hot" prong may vary between approximately +160 V and -160 V (.+-.110.times.2.sup.1/2). The neutral line stays at approximately 0 V relative to the "hot" line. The ground line is typically connected to an earth ground (approximately 0 V). Absent an improper "short circuit" between, for example, a hot wire in the appliance and the metal case of the appliance, the ground line does not carry substantial current.
When the hot line is at a positive voltage, source current flows along the hot line, through the electrical device, and then returns along the neutral line. Under normal operating conditions, with the electrical appliance operating properly, the current flowing through the hot line substantially equals the current returning along the neutral line.
It is possible, however, that a short may occur in the electrical appliance or along the remainder of the building circuit interconnected to the receptacle. The situation described might occur, for example, when a hot wire within the appliance becomes frayed and comes into contact with a metal casing on the electrical device. In such a case, the current flowing through the hot line may not return along the neutral line, but may, instead, flow to ground via an "improper" route. Such an improper route may be, for example, through the metal casing of the device and through the body of a human operator who is holding the device. This current is referred to as a "fault current."
Ground fault circuit interrupters, also referred to as GFCIs, are designed to quickly cut off the flow of current in an electrical circuit after the GFCI detects a sufficient fault current, thereby reducing the likelihood of electric shock. To function effectively, a GFCI must be capable of interrupting the electrical circuit within a relatively small time period after it detects a substantial ground fault.
Most GFCIs operate by detecting a difference between the level of current flowing through the hot line and the level of current flowing through the neutral line. This difference may be caused by a fault current improperly flowing to ground. When a fault current above a predetermined level is detected, a GFCI may respond, or "trip," interrupting the flow of current through the hot line.
Published industry standards describe the operation of some GFCIs. For instance, Underwriters Laboratories, Inc. has published Standard No. UL 943, which sets forth, in part, the maximum amounts of time in which a GFCI should trip in the presence of particular levels of fault current. More particularly, UL 943 includes a substantially exponential graph of GFCI-current versus trip time.
Ground fault circuit interrupters are often integrated together with standard power receptacles. The resulting single unit may be referred to as a receptacle ground fault circuit interrupter. Such a "receptacle GFCI" offers protection for users of devices plugged into the power receptacle.
Once a receptacle GFCI has been installed in a house or other building, it may be necessary to perform a variety of tests to ensure that the unit is properly connected to the local power circuit and further to ensure that the GFCI is operating properly. For example, an electrician installing or inspecting a receptacle GFCI may need to verify that the hot, neutral, and ground leads on the rear of the unit are properly connected, respectively, to the hot, neutral, and ground conducting lines of the local electrical power circuit. In the event any of these lines are reversed or are not connected, a power line may be absent or misconnected.
As another example, the electrician may need to accurately determine the voltage across the hot and neutral lines of the power receptacle. If the electrician is measuring the line voltage at a receptacle in the U.S. and determines that the voltage across the hot and neutral lines is substantially different that 110 V, RMS, the electrician may have reason to know that a problem exists in the building's electrical wiring.
Further, the electrician may need to determine whether the GFCI portion of the unit will function properly. Existing GFCI testing devices generally operate by shunting the hot line to the ground line in the power circuit, thereby causing current on the hot line to flow directly to ground, rather than flowing back along the neutral line. Sensing the resulting difference in current level between the hot and neutral lines, a properly functioning GFCI should promptly trip. If the GFCI fails to trip when thus tested, the GFCI may be defective and may need to be repaired or replaced.
Unfortunately, many existing GFCI test devices do not conveniently enable an electrician on site in a house or other building to determine whether the GFCI will trip quickly enough for a particular level of fault current. Thus, many portable GFCI testers do not determine whether the GFCI being tested trips within a proper time duration for a given level of fault current, such as those maximum time durations set forth in U.L. Standard UL 943.
Furthermore, presently available portable test devices for receptacle GFCIs often also fail to meet the additional needs of the on-site electrician mentioned above. For instance, existing portable equipment often fails to provide clear and easy-to-read indications of voltage and whether the hot, neutral, and ground leads of the receptacle unit are properly connected to, or disconnected from, the corresponding hot, neutral, and ground lines of the local electrical circuit. Additionally, some of the presently available electrical testing devices are cumbersome, heavy, and expensive.